[PATCH v2 3/3] vpci: allow 32-bit BAR writes with memory decoding enabled

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

Currently, Xen vPCI refuses BAR writes if the BAR is mapped in p2m. If
firmware initializes a 32-bit BAR to a bad address, Linux may try to
write a new address to the 32-bit BAR without disabling memory decoding.
Since Xen refuses such writes, the BAR (and thus PCI device) will be
non-functional.

Allow the hardware domain to issue 32-bit BAR writes with memory
decoding enabled. This increases the compatibility of PVH dom0 with more
hardware.

Note that Linux aims at disabling memory decoding before writing 64-bit
BARs. Continue to refuse 64-bit BAR writes in Xen while those BARs are
mapped for now to avoid mapping half-updated BARs in p2m.

Take the opportunity to remove a stray newline in bar_write().

Resolves: https://gitlab.com/xen-project/xen/-/issues/197
Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
---
v1->v2:
* rework on top of queued BAR map/unmap operation machinery

RFC->v1:
* keep memory decoding enabled in hardware
* allow write while memory decoding is enabled for 32-bit BARs only
* rework BAR mapping machinery to support unmap-then-map operation
---
 xen/drivers/vpci/header.c | 36 ++++++++++++++++++++++++------------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/xen/drivers/vpci/header.c b/xen/drivers/vpci/header.c
index 1c66796b625b..06c1dbfd5de0 100644
--- a/xen/drivers/vpci/header.c
+++ b/xen/drivers/vpci/header.c
@@ -404,9 +404,7 @@ static int modify_bars(const struct pci_dev *pdev, uint16_t 
cmd,
 
         if ( !MAPPABLE_BAR(bar) ||
              (rom_only ? bar->type != VPCI_BAR_ROM
-                       : (bar->type == VPCI_BAR_ROM && !header->rom_enabled)) 
||
-             /* Skip BARs already in the requested state. */
-             bar->enabled == !!(cmd & PCI_COMMAND_MEMORY) )
+                       : (bar->type == VPCI_BAR_ROM && !header->rom_enabled)) )
             continue;
 
         if ( !pci_check_bar(pdev, _mfn(start), _mfn(end)) )
@@ -650,19 +648,29 @@ static void cf_check bar_write(
         val &= PCI_BASE_ADDRESS_MEM_MASK;
 
     /*
-     * Xen only cares whether the BAR is mapped into the p2m, so allow BAR
-     * writes as long as the BAR is not mapped into the p2m.
+     * Allow 64-bit BAR writes only when the BAR is not mapped in p2m. Always
+     * allow 32-bit BAR writes.
      */
     if ( bar->enabled )
     {
-        /* If the value written is the current one avoid printing a warning. */
-        if ( val != (uint32_t)(bar->addr >> (hi ? 32 : 0)) )
-            gprintk(XENLOG_WARNING,
-                    "%pp: ignored BAR %zu write while mapped\n",
-                    &pdev->sbdf, bar - pdev->vpci->header.bars + hi);
-        return;
-    }
+        if ( bar->type == VPCI_BAR_MEM32 )
+        {
+            if ( val == bar->addr )
+                return;
 
+            modify_bars(pdev, pci_conf_read16(pdev->sbdf, PCI_COMMAND),
+                        VPCI_UNMAP, false);
+        }
+        else
+        {
+            /* If the value written is the same avoid printing a warning. */
+            if ( val != (uint32_t)(bar->addr >> (hi ? 32 : 0)) )
+                gprintk(XENLOG_WARNING,
+                        "%pp: ignored BAR %zu write while mapped\n",
+                        &pdev->sbdf, bar - pdev->vpci->header.bars + hi);
+            return;
+        }
+    }
 
     /*
      * Update the cached address, so that when memory decoding is enabled
@@ -682,6 +690,10 @@ static void cf_check bar_write(
     }
 
     pci_conf_write32(pdev->sbdf, reg, val);
+
+    if ( bar->enabled )
+        modify_bars(pdev, pci_conf_read16(pdev->sbdf, PCI_COMMAND), VPCI_MAP,
+                    false);
 }
 
 static void cf_check guest_mem_bar_write(const struct pci_dev *pdev,
-- 
2.50.1